Oil is a general term used to denote petroleum products which mainly consist of hydrocarbons. Crude oils are made up of a wide spectrum of hydrocarbons ranging from very volatile, light materials such as propane and benzene to more complex heavy compounds such as bitumens, asphaltenes, resins and waxes. Refined products such as petrol or fuel oil are composed of smaller and more specific ranges of these hydrocarbons.
Oil, when spilled at sea, will normally break up and be dissipated or scattered into the marine environment over time. This dissipation is a result of a number of chemical and physical processes that change the compounds that make up oil when it is spilled. The processes are collectively known as weathering. Oils weather in different ways. Some of the processes, like natural dispersion of the oil into the water, cause part of the oil to leave the sea surface, whilst others, like evaporation or the formation of water in oil emulsions, cause the oil that remains on the surface to become more persistent.
The way in which an oil slick breaks up and dissipates depends largely on how persistent the oil is. Light products such as kerosene tend to evaporate and dissipate quickly and naturally and rarely need cleaning-up. These are called non-persistent oils. In contrast, persistent oils, such as many crude oils, break up and dissipate more slowly and usually require a clean-up response. Physical properties such as the density, viscosity and pour point of the oil all affect its behaviour.
Dissipation does not occur immediately. The time this takes depends on a series of factors, including the amount and type of oil spilled, the weather conditions and whether the oil stays at sea or is washed ashore. Sometimes, the process is quick and on other occasions it can be slow, especially in sheltered and calm areas of water.
A number of models are available for predicting the trajectory and dispersion of oil spills at sea. These can serve as a useful guide to understanding how a particular oil is likely to behave and help in assessing the scale of the problem which a spill might generate.